Performance degradations such as a lower efficiency and a narrower bandwidth are expected when the physical size of an antenna becomes much smaller than the operating wavelength. As this is the case for most antennas operating in hearing aids or in similar SRD (Short Range Device) applications it is of great importance to optimize the antenna efficiency in order to keep the power consumption low. This is equally important as minimizing the size, so improving the efficiency of the antennas used in size critical battery operated instruments will result in a decrease in power consumption and a longer battery life. Challenges of antenna miniaturization are e.g. reviewed by [Skrivervik et al., 2001].
Recently published work [Alù et al., 2007] has shown that introducing a meta-material in a patch antenna structure can lead to the realization of 35 electrically small patch antennas presenting an unprecedented good efficiency. The combination of a normal dielectric material and a meta-material as substrate between the patch and the ground plane can support a cavity resonance with a frequency which is much lower than what can be expected from a conventional design. In addition to the small dimensions of the resonant structure, which can also be achieved with a high permittivity dielectric material, the meta-material maintains good radiation efficiency. In contrast to the high permittivity dielectric material which traps most of the energy inside the material the meta-material sets up means to fulfil the resonant boundary conditions within small dimensions, and allows the electromagnetic fields to extend outside the structure.